Chemistry 105 Exam 2 Homework-Monte Helm

Given the following equation, 4 KClO3⟶4 KCl+6 O2, how many moles of O2 can be produced from 8.60 moles KClO3?

12.9 moles. Explanation: The ratio of O2 to KClO3 to is 6:4. Therefore, 8.60mol KClO3×6 mol O2/4mol KClO3=12.9 mol O2.

What is the mass, in grams, of 2.88 moles of glycine (C2H5NO2)? Your answer should have three significant figures.

216 grams. Explanation: Recall that the formula mass of an element or compound gives the mass of one mole of that substance. Convert the moles of C2H5NO2 to grams by multiplying by the formula mass of C2H5NO2 found with the Periodic Table: 2.88 moles×(75.067 grams/1 mole)=216.19 g. The answer should have three significant figures, so round to 216 g.

In the equation 5Z→2A+3B, how many molecules of Z must react to give 15 molecules of B?

25 molecules of Z. Explanation: If we expect to get five times as many molecules of B as are listed in the chemical equation, we must also multiply the number of molecules of Z by five, and 5×5=25.

How many atoms are in a 36 g sample of iron? Select the correct answer below: A. 3.88×10^23 atoms B. 1.21×10^27 atoms C. 8.34×10^23 atoms D. 5.64×10^26 atoms

A. 3.88×10^23 atoms. Explanation: To calculate the number of atoms in a sample of a given mass, we divide by the molar mass and multiply by Avogadro's number (36g Fe×1mole Fe/55.845 g Fe×6.022×10^23/1 mole=? atoms). We can see that using dimensional analysis, units of grams and moles will be cancelled out, and the desired units of atoms are left in the equation. Performing the calculation and solving for the final answer, we get: 36×1/55.845×6.022×10^23/1=3.88×10^23 atoms.

How many grams are in 3.30 moles of potassium sulfide (K2S)? Select the correct answer below: A. 364 grams B. 360. grams C. 362 grams D. 36.4 grams

A. 364 grams. Explanation: To determine the number of grams of a substance, we multiple the number of moles by the molar mass. Grams=moles×molar mass=3.30 moles×110.262g/mol=363.8646 g. Rounding to three significant figures, the mass is equal to 364 g K2S.

How many atoms of aluminum are in 0.250g of aluminum? Select the correct answer below: A. 5.58×10^21 atoms B. 4.06×10^24 atoms C. 9.27×10^−3 atoms D. 6.02×10^23 atoms

A. 5.58×10^21 atoms. Explanation: Dividing mass by molar mass yields number of moles. Multiplying moles by Avogadro's number yields number of atoms. 0.250gAl×1molAl/26.982gAl×6.022×10^23atoms Al/ 1molAl=5.58×10^21 atoms Al.

How many grams of sodium chlorate are required to generate 50.0 g sodium chloride according to the following equation: 2NaClO3→2NaCl+3O2. Select the correct answer below: A. 91.1 g B. 102 g C. 45.5 g D. 16.3 g

A. 91.1 g. Explanation: The coefficients in the balanced chemical equation can be used as the stoichiometric factor between NaCl and NaClO3. Molar mass converts between grams and moles. 50.0 g NaCl(1 mole NaCl/58.443 g NaCl)(2 moles NaClO3/2 molesNaCl)(106.44 g NaClO3/1 mol NaClO3)=91.1 g NaClO3.

Using the chemical equation provided, identify which of the following are the reactants. 4Fe+3O2⟶2Fe2O3 Select all that apply: a. Fe b. O2 c. Fe2O3 d. All of the above

A. Fe and B. O2. Explanation: In a chemical equation, the reactants are always to the left of the arrow. In this reaction the reactants are Fe and O2. Fe2O3 is to the right of the arrow and is therefore a product in this reaction.

Which of the following is used to determine stoichiometric factors? Select the correct answer below: A. coefficients in a balanced chemical equation B. coefficients in an unbalanced chemical equation C. total number of reactants in a chemical equation D. total number of products in a chemical equation

A. coefficients in a balanced chemical equation.

Avogadro's number is properly reported with an explicit unit of? Select the correct answer below: A. per mole B. per atom C. per molecule D. per coefficient

A. per mole. Explanation: Avogadro's number is properly reported with an explicit unit of "per mole," a conveniently rounded version being 6.022×1023/mol. Depending on the substance that we are considering, we will add the appropriate units to the numerator of Avogadro's number when we use it in calculations.

Substances that are present before a chemical reaction occurs are called: Select the correct answer below: A. reactants B. products C. intermediates D. catalysts

A. reactants. Explanation: Reactants are present before a chemical reaction occurs because these are the substances that react with one another in the chemical reaction.

From the following balanced equation, CH4(g)+2O2(g)⟶CO2(g)+2H2O(g), how many grams of H2O can be formed from 1.25g CH4? Select the correct answer below: A. 1.25g B. 2.81g C. 1.41g D. 0.702g

B. 2.81g. Explanation: The coefficients in the balanced chemical equation can be used as the stoichiometric factor between CH4 and H2O. Molar mass converts between grams and moles. 1.25gCH4×1molCH4/16.043gCH4×2molH2O/1molCH4×18.015gH2O/1molH2O=2.81gH2O.

What is the percent yield of NH3, if the following reaction is performed using 77 g CaCN2 and produces 27.1 g NH3? CaCN2+3H2O-->CaCO3+2NH3. Select the correct answer below: A. 76.4% B. 82.9% C. 89.4% D. 93.2%

B. 82.9%. Explanation: After verifying that the equation is balanced, we calculate the theoretical yield: 77 g CaCN2/80.102g CaCN2/mol CaCN2×2 mol NH3/1 mol CaCN2×17.031 g NH3/1 mol NH3=32.7 g NH3. Comparing the theoretical yield to the actual yield (27.1 g), we obtain percent yield as follows: 27.1 g/32.7 g×100=82.9%.

Identify the reaction where a single compound separates into multiple compounds or elements. Select the correct answer below: A. displacement reaction B. decomposition reaction C. double displacement reaction D. combination reaction

B. decomposition reaction.

Acid-base gas evolution reactions come under which type of reactions? Select the correct answer below: A. displacement reaction B. double displacement reaction C. decomposition reaction D. combination reaction

B. double displacement reaction. Explanation: Acid-base gas evolution reactions involve a double displacement reaction that forms water and an unstable molecule that then decomposes into a gas and a stable compound.

A _________ is defined as the amount of substance containing the same number of discrete particles as the number of atoms in a 12-gram sample of pure carbon. Select the correct answer below: A. neutron B. mole C. mass unit D. proton

B. mole

Substances that are only present when a chemical reaction is complete are called_______. Select the correct answer below: A. reactants B. products C. intermediates D. catalysts

B. products. Explanation: Products are present after a chemical reaction, because these are the substances that are produced once the reactants react with one another.

A chemical equation depicts substances__________. Select the correct answer below: A. in alphabetical order B. undergoing a change C. in increasing quantities D. in decreasing quantities

B. undergoing a change. Explanation: A chemical equation depicts substances undergoing a change. This change involves breaking and forming of bonds to create new substances. An example is the reaction between one methane molecule (CH4) and two diatomic oxygen molecules (O2) to produce one carbon dioxide molecule (CO2) and two water molecules (H2O). CH4+2O2→CO2+2H2O.

What is the formula mass for NaBr (sodium bromide)? Select the correct answer below: A. 74.193 amu B. 99.821 amu C. 102.894 amu D. 103.211 amu

C. 102.894 amu. Explanation: The formula mass of NaBr is 1(22.990)+1(79.904) which is 102.894 amu.

If two compounds in a chemical reaction are listed with the coefficients 2 and 5 respectively, which is NOT a possible numerical combination of these compounds that will result in their complete reaction? Select the correct answer below: A. 4 and 10 B. 100 and 250 C. 8 and 18 D. 40 and 100

C. 8 and 18. Explanation: Any numerical combination of these compounds that results in a complete reaction will reduce to a 2:5 ratio. 8 and 18 is the only option that does not obey the explicit 2:5 ratio dictated by the coefficients in the chemical equation. All of the other values reduce to a 2:5 ratio.

Which sample has the most oxygen atoms? Select the correct answer below: A. a mole of CO B. a mole of H2O C. a mole of CO2 D. these all have the same number of oxygen atoms

C. a mole of CO2. Explanation: Since there are two oxygen atoms per molecule, there will be twice as many oxygen atoms in the sample as there are in the other two compounds.

The relative numbers of reactants and products are represented by: Select the correct answer below: A. products B. reactants C. coefficients D. subscripts

C. coefficients. Explanation: Coefficients, which are numbers placed to the left of each compound in the equation, indicate the relative numbers of reactants and products that react.

Converting from moles of one compound into moles of another compound in a chemical equation will require the______. Select the correct answer below: A. molar masses B. molecular masses C. stoichiometric factor D. molecular geometries

C. stoichiometric factor. Explanation: The stoichiometric factor between two compounds will allow us to convert from the moles of one to the moles of the other.

The reactant that is almost entirely consumed in a reaction, thus limiting the amount of product generated, is known as: Select the correct answer below: A. the balancing reactant B. the excess reactant C. the limiting reactant D. none of the above

C. the limiting reactant.

In a chemical equation, the number of atoms of a particular element that are present in a substance is indicated by a _____. Select the correct answer below: A. coefficient B. superscript C. plus sign D. subscript

D. subscript. Explanation: Subscripts indicate how many atoms of a particular element are present in a substance. Coefficients tell us how many molecules of a substance are involved in the reaction and superscripts indicate a charge on an element or substance. A plus sign separates multiple reactants or products in a chemical equation.

How many grams are in 11.9 moles of chromium? Your answer should have three significant figures.

619 grams. Explanation: To determine the number of grams of a substance, we multiple the number of moles by the molar mass. Grams=moles×molar mass=11.9 moles×51.996g/mol=618.7524 g. Rounding to three significant figures, the mass is equal to 619 g Cr.

How many sulfur atoms are there in 5.325×10−23 grams of sulfur? Use 6.022×1023mol−1 for Avogadro's number. Your answer should be a whole number without any decimal places.

1 atom. Explanation: First, convert the mass of S into moles by dividing by the molar mass of S found on the Periodic Table: 5.325×10^−23 g×1 mol S/32.066 g=1.6606×10^−24 mol. Recall that Avogadro's number describes the number of atoms or molecules that make up a mole. By dimensional analysis, we see that we can convert moles into atoms by multiplying the moles of S by Avogadro's number: 1.6606×10^−24 mol×6.022×10^23 atoms/1 mol=1.00004 atom. Since there can only be a whole number of atoms, the rounded answer of 1 atom can be treated as exact.

Given the following equation, 2 KClO3⟶2 KCl+3 O2, how many moles of O2 can be produced from 12 moles KClO3? Report your answer as a whole number without any decimal places.

18 moles. Explanation: The ratio of O2 to KClO3 is 3:2. Therefore, 2mol KClO3×3 mol O2/2mol KClO3=18 mol O2.

What is the mass in grams of 1.00×1023 radon atoms? Use 6.022×1023mol−1 for Avogadro's number. Your answer should have three significant figures.

36.9 grams. Explanation: Recall that Avogadro's number gives the number of atoms or molecules that make up a mole. By dimensional analysis, we see that we can convert the number of atoms into moles by dividing by Avogadro's number: 1.00×10^23 atoms×1 mol/6.022×10^23 atoms=0.1661 mol. Next, convert the moles of radon atoms into grams, using the molar mass found on the Periodic Table: 0.1661 mol×222.018 g/ 1 mol=36.9 g.

In the equation 4A+2B→3C, if 12 molecules of A react, how many molecules of C will we get? Report your answer as a whole number without any decimal places.

9 molecules. Explanation: The coefficient ratio between A and C is 4:3. If 12 molecules of A react, this is three times the coefficient of A. To get the corresponding number of C molecules we must also triple the coefficient on C. This gives a result of 9 molecules of C produced.

What is the symbolic representation of a chemical reaction with identities and relative quantities known as? Select the correct answer below: A. a chemical equation B. a molar mass C. a stoichiometric formula D. a molar percentage

A. a chemical equation.

How many moles are in 15 g of carbon dioxide (CO2)? Select the correct answer below: A. 0.28 moles B. 0.34 moles C. 0.32 moles D. 0.29 moles

B. 0.34 moles. Explanation: First we need to determine the molar mass of carbon dioxide. Since we know that the formula is CO2 we can easily determine this using the periodic table of elements. Carbon has an atomic mass of 12.011 and oxygen has an atomic mass of 15.999 (don't forget that the subscript tells us that there are two oxygens!). Therefore, to determine the total molar mass for the compound we add the molar masses of carbon and oxygen: 12.011+(2×15.999)=44.009. To then determine the number of moles in 15 g of carbon dioxide, we need to use the formula: mass of CO2 atoms (g)/molar mass of CO2(g/mol)=moles of CO2. Therefore, 15 g CO2/44.009 g/mol=0.34 moles CO2.

What is the average mass in grams of one uranium atom? Use 6.022×1023 atoms per mole for Avogadro's number. Select the correct answer below: A. 2.383×10^−24g B. 1.000×10^−22g C. 3.953×10^−22g D. 2.387×10^23g

C. 3.953×10^−22g. Explanation: To calculate the mass, in grams, of one uranium atom, we divide by Avogadro's number and multiply by the molar mass: 1 atom×1 mole/6.022×10^23atoms×238.029 g/1 mole=3.953×10−22 g.

How many grams of helium are in a balloon with 8.40 moles of helium atoms? Select the correct answer below: A. 2.13 g B. 12.4 g C. 33.6 g D. 840. g

C. 33.6 g. Explanation: Recall that the formula mass of an element or compound gives the mass of one mole of that substance. Convert the moles of He to grams by multiplying by the formula mass of helium found on the Periodic Table: 8.40 moles×(4.003 grams/1 mole)=33.625 g. The answer should have three significant figures, so round to 33.6 g.

If a compound's chemical formula was experimentally determined to be 18, the chemical formula could be_____. Select the correct answer below: A. CH4 B. NH3 C. H2O D. PCl5

C. H2O. Explanation: Use the periodic table to get the atomic masses of each element. Then you can calculate the formula mass as such: H=1.008×2=2.016 O=15.999×1=15.999 Adding these together we get 18.015, which is approximately 18.

How many helium atoms are in a 12.0 g sample? Select the correct answer below: A. 48.0 atoms B. 4.00×10^19 atoms C. 1.20×10^23 atoms D. 1.81×10^24 atoms

D. 1.81×10^24 atoms. Explanation: To determine the number of atoms in a 12.0 g sample of helium, we must divide by the molar mass and multiply by Avogadro's number. 12.0 grams×1 mole/4.003 grams×6.022×10^23 atoms/1 mole=1.81×10^24 atoms

If the theoretical yield of a reaction is 100 grams, which value for actual yield is physically impossible? Select the correct answer below: A. 0 grams B. 50 grams C. 99.9 grams D. 110 grams

D. 110 grams. Explanation: It is physically impossible to generate an actual yield greater than the theoretical yield as that would involve the spontaneous generation of new matter, or at the very least a high amount of experimental error.

What is the mass of 4.45×1022 tin (Sn) atoms? Use 6.022×1023atoms/mole for Avogadro's number. Select the correct answer below: A. 118g B. 60.2g C. 44.5g D. 8.77g

D. 8.77g. Explanation: To calculate the mass of 4.45×1022 tin atoms, use Avogadro's number to calcuate the number of moles, and the molar mass of tin to convert moles to mass in grams: 4.45×10^22 atoms×1 mo/l6.022×10^23 atoms×118.711 g/1 mol=8.772 g. After rounding to three significant figures, we find that 4.45×1022atoms of tin have a mass of approximately 8.77g.

________ is the extent to which a reaction's theoretical yield is achieved. Select the correct answer below: A. Thermal chemical yield B. Actual yield C. Theoretical yield D. Percent yield

D. Percent yield.

Sodium azide, NaN3(s), is responsible for the rapid deployment of air bags during a car crash. The reaction is initiated by an electrical discharge and produces nitrogen gas and elemental sodium. This is an example of which type of reaction? Select the correct answer below: A. displacement reaction B. combination reaction C. double displacement reaction D. decomposition reaction

D. decomposition reaction. Explanation: one reactant, sodium azide (NaN3), is separated into two products, Na and N2.

Which sample has the greatest mass of hydrogen? Select the correct answer below: A. a mole of H2O B. a mole of H2S C. a mole of H2Se D. these all have the same mass of hydrogen

D. these all have the same mass of hydrogen. Explanation: Since these all have the same number of hydrogen atoms, they will all contain the same mass of hydrogen.

Which of the following reactions is stoichiometrically equivalent to the reaction shown below? Pb(OH)4+2H2SO4→Pb(SO4)2+4H2O Select the correct answer below: a. 2Pb(OH)4+4H2SO4→2Pb(SO4)2+4H2O b. 2Pb(OH)4+4H2SO4→2Pb(SO4)2+8H2O c. Pb(OH)4+4H2SO4→2Pb(SO4)2+8H2O d. Pb(OH)4+2H2SO4→3Pb(SO4)2+4H2O

b. 2Pb(OH)4+4H2SO4→2Pb(SO4)2+8H2O. Explanation: The given equation has a coefficient ratio of 1:2→1:4. Answer B maintains this ratio with 2:4→2:8, which reduces to 1:2→1:4.

If a compound's formula mass was experimentally determined to be 76 amu, the chemical formula could be________. Select the correct answer below: a. AgNO3 b. H2O c. NO d. N2O3

d. N2O3. Explanation: The formula mass for N2O3 is calculated by 2(14.007)+3(15.999)=76.011 amu which is approximately 76 amu.